DESCRIPTION: This project will examine the cellular basis of signal transformations between mossy fibers and neurons within the granular layer of the vestibular cerebellum. The signal transformations which occur between first order mossy fiber projections and neurons of the granular layer vestibular cerebellum (nodulus, uvula) represent a critical step in the cerebellar regulation of vestibular reflexes. Recently, a new cell type within the granular layer densely concentrated in vestibular areas of cerebellum has been identified, termed unipolar brush cells (UBCs) and electron microscope, immunocytochemical and patch-clamp recording studies have revealed these neurons to have unique ultrastructural and synaptic properties. UBCs are generally innervated by a single glutamatergic mossy fiber at a synapse which has unusually extensive region of synaptic contact, and the predominant excitatory postsynaptic response to mossy fiber activation is a long-lasting synaptic conductance whose biophysical properties of transmission are reflected in the ultrastructural features of the synapse. Furthermore, UBCs are excitatory and have axons which branch within the granular layer to give rise to rosette-like presynaptic terminals, thus UBCs represent interneurons of the granular layer providing powerful feedforward excitation within cerebellum. Experiments to further study the biophysical and pharmacologic properties of this grant glutamate receptor-mediated synapse will be conducted using whole- cell, patch-clamp recording in thin slices of rat vestibulocerebellum maintained in vitro. The contributions of receptor desensitization, transmitter re-uptake, and single-channel properties to the time course of the prolonged synaptic response of UBCs to mossy fiber stimulation will be examined using both whole-cell and excised patch recordings. The spatial and temporal dynamics of changes during the synaptic current will be examined using the combined application of whole-cell recording and calcium imaging methods in thin cerebellar slices. The time course and origin of changes in intracellular calcium will be compared to that of synaptic current. The mossy fiber-UBC synapse displays both short-term and long-term forms of synaptic plasticity which will be of consequence to temporal synaptic integration within cerebellum, and may contribute to long-term changes in vestibular reflex modulation. The cellular mechanisms of synaptic plasticity will be examined in UBCs to determine the contribution of pre- and postsynaptic receptors and their underlying intracellular messenger cascades using both patch-clamp recording and calcium imaging approaches. These techniques will also be used to determine the identity of the target neurons of UBCs within the granular layer, and the study of the time course and cellular mechanisms of transmission between UBCs and their target neurons to further understand how this unique class of neurons within the cerebellum integrates afferent input. These experiments will provide a comprehensive picture of the cellular neurobiology of UBCs which will serve as essential prelude to understanding their contribution to the control of motor reflexes i the vestibular cerebellum.